A review of the American radiologic literature failed to disclose any account of malignant mediastinal teratoma. Dann (1) and Present (3) have reported single cases of benign mediastinal teratoma, and Rusby (4) reviewed 245 cases of intrathoracic dermoid cysts and teratomas. The pathology and etiology of mediastinal teratomas are well discussed by Schlumberger (5) and Willis (6). Schlumberger presented 16 cases, 6 of which were malignant. One of his patients showed metastatic invasion of the twelfth thoracic vertebra. In Rusby's series, 12.9 per cent of the teratomas were malignant. A teratoma is a true neoplasm, composed of multiple tissues foreign to the part of the body in which it arises and possessing the power of progressive growth. According to its behavior or structure, it may be benign or malignant. Clinically, malignant mediastinal teratomas are seen predominantly in males. The average age of occurrence is twenty-seven years. After onset of symptoms, the average duration of life in Schlumberger's series was five and a half months. Presenting complaints included cough, chest pain, dyspnea, hemoptysis, and engorgement of the neck veins. Hammarskjöld (2) and Rusby have discussed some of the radiologic features of malignant mediastinal teratoma. The tumor is usually a large, solid-appearing, homogeneous mass located in the anterior mediastinum. With growth it extends superiorly, inferiorly, and laterally, often with localization to one side or the other. The border is usually quite distinct and lobulated. The identification of teeth, bone, or fatty tissues is uncommon. In contrast, benign mediastinal teratomas are typically cystic in appearance, have a smooth, round border, and frequently contain teeth, bone, and fatty tissues. The malignant transformation of a benign tumor is rarely heralded by any sudden increase in size. Case Report An 18-year-old white male was first seen at The Henry Ford Hospital on Jan. 8, 1960, because of fullness and pain in the right upper quadrant of about one year duration. He complained also of intermittent severe pain in the sacral region and right upper leg and for the past three weeks had experienced pain in the right lower chest and moderate dyspnea. He denied cough, hemoptysis, or fever. The patient appeared thin, pale, and chronically ill. The right anterior chest wall was bulging. There were dullness to percussion and râles over the right lung base. Marked voluntary guarding of the right upper quadrant abdominal muscles was noted. The area over the coccyx was tender, but there were no pathologic reflexes or limitation of motion. Laboratory studies on admission were normal, with the exception of a white blood cell count of 10,400. Admission roentgenograms (Fig. 1) revealed extensive opacification of the right hemithorax, with only a small amount of aerated lung in the apical region.